1-Effect of varying THI on DBG and DMI:
The results indicated that the rate of decline in DBG and DMI increases with increasing THI values. DBG in native bovine calves decreased significantly by 18.6% at THI 74.9 during the spring season and by 41.1% at THI 85.5 during the summer season compared to THI 68.1 during the winter season. These results indicate that the most deteriorating effect of warmth stress on both DBG and DMI was observed in the summer season. Nardone et al. (1993) found that 86 THI from 10.00 to 18.00 hours for ten weeks determined a decrease in DMI (7.9%) and a reduction in DBG (26.1%) in Holstein Friesian calves. West (1993) reported that the range of THI values from 71 to 81 caused significant decreases in the consumption of TDN of protein in cattle. A decrease of 0.57, 0.51, 0.48, 0.42 and 0.29 kg DMI daily for every unit rise in THI for Asia, South America, Oceania, Europe and North America, respectively (Chang-Fung-Martel et al., 2021). West et al. (2003) found that THI deleteriously affected feed intake, where a THI>72.1 resulted in an exceedingly decrease in the feed intake of Holstein and Jersey cows by 0.51 kg and 0.47 kg, respectively, for every unit of increase in THI within the range of 72 to 84. Bouraoui et al. (2002) found that the average DMI reduced significantly by 9.6% (1.73 kg) from 18.0 to 16.27 kg/day in lactating Friesian-Holstein cows when THI increased from 68 in spring to 78 in summer. Calves suffering a mean THI <50 gained more (0.67 kg/d) than calves suffering an average THI from 50 to 69 (0.62 kg/d), or ≥70 (0.59 kg/d) (Shivley et al., 2018). Higher mean THI was related to a lower DBG at the calf market, and the DBG values at THI of >75 were significantly below those at THI of ≤50 or THIs starting from 56 to 60 (Nabenishi and Yamazaki, 2017). Xue et al. (2010) confirmed that the DMI values increased from 18.5 to 19.8 kg daily when THI increased from 42 to 68% and then decreased from 19.8 to 15.8 kg daily when the THI increased from 68 to 80%. The authors concluded that DMI increases slowly with the rise in THI until the critical point and decreases severely with the rise THI. Habeeb et al. (2018 a, b) reported that the important effect of warmth stress on livestock is the decrease of feed intake to provide less metabolic heat and eventually reduced growth by quite 10-20% (Habeeb et al., 2018 a, b). The DMI of animals generally begins to decrease when the ambient temperature reaches 25°C and severely decreases when the environmental temperature exceeds 40°C, after which DMI is around 20–40% less than the regular intake (Habeeb et al., 2018c).
The depression in DMI and growth performance of animals during heat stress may be due to redistributing energy to heat regulation through a series of physiological and metabolic responses, like elevated blood insulin and protein catabolism, enhanced respiration rate, and panting accelerate the loss of CO2, leading to altered blood acid-base chemistry and alkalosis (Wang et al., 2020). Besides, the high THI prompted some alterations in rumen motility and microbiota which affects feed digestibility and rumen fermentation, causing the change of feed digestibility and rumen fermentation (Wang et al., 2020). Under high ambient temperatures, livestock is predicted to decrease DMI to cut back their metabolic heat production. Reduced internal secretion concentration in heat-stressed animals may result in reduced feed intakes which results in deleterious energy balance making energy levels not adequate for normal growth synthesis (Habeeb et al., 2020).
2-Effect of varying THI values on water consumption (WC):
The increase in water consumption (WC) increases with increasing THI values. WC significantly in native bovine calves increased by 28.8% at THI 74.9 during the spring season and by 63.4% at greater THI 85.5 during the summer season compared to those at low THI 68.1 in winter.
Nardone et al. (1993) found that 86 THI from 10.00 to 18.00 hours for ten weeks determined an increase in WC (29.1%) in Holstein Friesian calves. Kadzere et al. (2002) observed that WC augmented by 3.3 liters per day when THI elevated from 70.01 to 87.72%. In a controlled chamber for seven days on Korean calves, Kim et al. (2018) observed that WC was significantly increased at THI 82.92 to 84.05 compared to that at THI 74.22. Habeeb et al. (2018a) found that WC was increased by 30% or more during heat stress conditions compared to that in the absence of heat stress conditions.
The most effective way of heat dissipation when animals are exposed to high-temperature is increasing the rate of evaporative heat exchange. The loss of water by sweating and stimulates water consumption (Kadzere et al., 2002).
3-Effect of varying THI on food conversion (DMI/DBG), DMI/WI and DBG/WI:
The efficacy of conversion of DMI to DBG was increased significantly by 13.1% during the spring season and by 33.6% during the summer season compared to those in winter. With increasing THI, the depression in DBG was more than the decline in daily DMI; therefore, food conversion (DMI/DBG) increases when the value of THI increases.
DMI/WC ratio decreased during the spring season by 27.8% and during the summer season by 51.7% compared to the winter season. The percentage decline in DMI was less than the percentages increase in WI; therefore, the DMI/WC ratio decreased significantly with increasing THI values.
The Gain/WC ratio decreased significantly with increasing THI levels. Gain/WC ratio decreased significantly by 34.7 and 64.0% during the spring and summer seasons, respectively, compared to during the winter season. The share decline in WC was less than the odds increase in gain; therefore, the Gain/WI ratio decreased significantly with increasing THI values. The results of the DMI/WI ratio indicates that at low THI (THI <70) in the winter season, each 2.09 g DMI need to 100 ml drinking water while at THI (70< - >80) in the spring season, each 1.51 g DMI need 100 ml water while at greater THI (80<) in the summer season, each 1.01 g only need 100 ml drinking water.
The Gain/WI ratio also decreased significantly with increasing THI levels. Gain/WI ratio decreased significantly by 34.7 and 64.0% during the spring and summer seasons, respectively, compared to during the winter season. Each 3.08 g gain at low THI (THI <70) needs 100 ml drinking water while at THI 74.9, each 2.01 g gain needs 100 ml water, and at greater THI 85.5, each 1.11 g only need 100 ml drinking water.
The utilization of feed efficiency indexes aims to spot and choose animals with a significant economic amount, and feed efficiency would vary with an index of THI (Veerkamp, 1998). The regression of y on x indicates that when the worth of THI increases the food efficiency per kg food increases (Könyves et al. 2017). An animal's efficiency in converting feed into products is influenced by environmental factors that lead to individual variation in energy expenditure (Habeeb et al., 2018a).
4-Effect of varying THI values on hormonal levels:
Thyroid hormones decreased significantly and cortisol increased significantly with increasing THI. T4 and T3 levels in native bovine calves were significantly decreased by 8.22 and 13.2% during the spring season and were significantly decreased by 24.2 and 33.8%, respectively, during summer compared with those values during the winter season. The results showed that with increasing THI, the decrease in T3 was over the decline in T4; therefore, the T4/T3 ratio increases significantly when the worth of THI rises. The cortisol hormone levels were increased significantly by 28.5 and 61.4 during the spring and summer seasons, respectively, compared with the winter season. These results indicate that the foremost deteriorating effect of warmth stress on hormonal levels was observed within the summer season. During the warmth stress conditions, O’Brien et al. (2010) found that T4 and T3 levels in growing cattle are decreasing significantly compared with THN conditions. Silanikove (2000) found that T3 and T4 concentrations in domestic ruminants decreased by 25% due to heat stress conditions. When the THI was higher, the thyroid hormones levels in blood serum were below the conventional range and when the THI was in comfort, the T3 and T4 levels in body fluid were increased (Kohli et al., 2014). The study of Wankar et al. (2019) was conducted on adult buffaloes and found that thyroid hormones declined and glucocorticoids increased at 35°C and 40°C. Kim et al. (2018) found that at THI of 84.05 to 87.72, serum cortisol level was significantly higher than at THI of 70.01 to 82.92. The decline in T4 and T3 levels at THI is consistent with the decline in metabolic rate, feed intake, growth and milk production (Agarwal and Upadhay, 2013). Exposure of the Indian goats to high environmental temperature adversely affects the endocrine activity because of its high sensitivity to environmental heat variations, consequently resulting in declined T3 and T4 levels (Sejian and Srivastava 2010). The decline in thyroid activity is combined with a decrease in hormone to scale back metabolic activity and body heat increment. The hypothalamus-pituitary-adrenal axis stimulation results in the rise within the secretion of corticotropin from the pituitary and consequently starts the synthesis and secretion of cortisol hormone (Lakhani et al., 2018; Li et al., 2020).
5- Effect of varying THI values on blood biochemical components:
The concentrations of total protein and glucose within the calves decreased significantly with increasing THI. During the summer season, the concentrations of total protein and glucose were significantly lower by 15.1 and 15.7%, respectively than those values during the winter season. Levels of plasma glucose and total proteins in the blood of Egyptian native calves at low and moderated THI levels were maintained relatively stable, while a significant reduction was observed at high THI.
The urea concentration decreased significantly with increasing THI. Under the spring season, urea levels in bovine calves increased significantly by 18.3%, while under the summer season, urea levels increased significantly by 64.2% compared with under the winter season.
The serum glucose level at THI of 82.92 was significantly higher than at THI of 70.01 to 76.51(Kim et al. 2018). The reduced energy intake as a result of reduced feed intake and increased cost for the thermoregulation process may be caused by the decrease in serum glucose level at high THI. The negative effect of the energy at high THI may be increased gluconeogenesis as an endocrine acclimation to hot conditions (Abeni et al. (2007). In Egyptian goats, El-Tarabany et al. (2017) found the same reduction in glucose and total protein concentrations at high THI levels as compared with low and moderate THI levels and clarified that decline to the augmentation in the plasma volume as a sequel of warmth shock which ends in decreased protein concentration.
6. Correlations coefficient between THI with different parameters:
THI has highly significant negative correlations with DBG, DMI, T4, T3, total proteins, and glucose and has highly significant positive correlations with water intake also as cortisol and urea concentrations. DBG has highly significant positive correlations with DMI, T4, T3, total proteins and glucose and has highly significant negative correlations with water intake, cortisol and urea concentrations. DMI has highly significant positive correlations with T4, T3, total proteins, and glucose and has highly significant negative correlations with water intake, cortisol, and urea concentrations. THI had a significant negative correlation with the DMI of cows in the south-eastern of the US (Holter et al., 1997).
THI had a significant negative correlation (r = - 0.82) with the DMI of cows (Chang-Fung-Martel et al., 2021). Davina and Eileen (2017) reported that THI had negative effects on daily DMI, daily CP intake, daily ME intake and feed efficiency. THI had a positive correlation with WC in lactating dairy cows (Ammer et al., 2018). The same authors mentioned that WC increased between 0.96 and 1.08 liter per rising THI one unit. El-Tarabany et al. (2017) found that THI values were negatively correlated with serum glucose and total protein. Li et al. (2020) found that negative significant correlations of THI with cortisol 0.624 and located also that cortisol was positively significantly correlated with T4 (0.814). Kim et al. (2018) found a significantly positive association between THI and cortisol. THI had a positively correlation with cortisol and thyroxin hormonal levels (Bouraoui et al., 2002).